Shapiro delay caused by nearby white dwarf companion
Using a technique that exploits general relativity, astronomers in the US and the Netherlands have identified a neutron star that has a mass nearly twice that of the Sun. The object is the most massive neutron star ever determined with reliable precision, and its existence, say the researchers, rules out much of the exotic matter hypothesized to occur inside these ultra-dense burnt out stars.
Neutron stars form when stars exhaust all of their nuclear fuel and implode under their own weight. The immense gravity forces protons and electrons together, leaving a ball made up largely of neutrons that has a density up to ten times that of atomic nuclei. Theorists have proposed that the huge pressure inside such an object could lead to a number of different forms of exotic matter. One of these is a material known as a Bose–Einstein condensate, in which particles act together as a single quantum entity. Alternatively, the neutrons might split to form a “soup” of free quarks and create what is known as a “quark star”.
Read more here physicsworld.com.